The present invention relates to an image forming elements array for use in a digital image writing optical system of image forming apparatuses such as digital copying machines, printers and facsimile machines, and an optical printing head and an image forming apparatus using the image forming elements array.
Recently, because image forming apparatuses such as digital copying machines, printers and facsimile machines must be smaller, digital image writing apparatuses included in such image forming apparatuses must also be smaller. Generally, digital image writing systems are classified into two types: (1) an optical scanning system which scans a light flux emitted from a light source such as a semiconductor laser by deflecting the light flux with a deflector and form the deflected light flux into an optical spot with a scanning image forming lens; and (2) a solid scanning system which forms a light flux emitted from a light emitting elements array such as an LED array into an optical beam spot with an image forming elements array.
The optical scanning system requires a relatively long light path due to the use of an optical deflector to deflect a light flux from a light source so as to scan a scanned surface. Therefore, the size of an apparatus using a digital image writing apparatus including the optical scanning system is relatively large. On the other hand, the solid scanning system can make a light path relatively short, and thereby an apparatus using a digital image writing apparatus including the solid scanning system can be made relatively small. Further, the digital scanning system has advantage in that mechanical devices that have to be driven, such as an optical deflector, are not required.
Japanese Patent Laid-open Publication No. 10-153751 describes an example of such digital image writing apparatuses using a solid scanning system. As illustrated in FIG. 1(A), an image forming elements array 90 included in the digital image writing apparatus includes a plurality of image forming elements which are arranged in an arrangement direction. Each image forming element has an incident surface 90a at the incident side, i.e., the light emitting elements array side, an emerging surface 90b at the emerging side, i.e., the scanned surface side, and a pair of reflective surfaces 90c, which are formed integrated with each other. The pair of reflective surfaces 90c are arranged at right angles relative to each other so as to define a roof-prism. The reflective surfaces 90c do not affect an imaging function and are slanted 45 degrees relative to the incident light axis.
A light flux emitted from a point of the surface of a light emitting element is transmitted to the incident surface 90a of the image forming elements array 90, is sequentially reflected by the pair of reflective surfaces 90c and then emerges from the emerging surface 90b to reach a scanned surface of a photoconductor. The incident light axis and the emerging light axis are substantially at right angles to each other. An image at the point of the light emitting element surface is formed by the image forming function of the incident surface 90a and the emerging surface 90b at a corresponding point of the scanned surface of the photoconductor.
The image forming elements array 90 further includes, as illustrated in FIG. 1(B), an aperture member 91 having a plurality openings 91a for optimizing the light quantity and for preventing cross talk between the adjacent image forming elements. The aperture member 91 has a plate-like shape having an L-shaped configuration in the cross-section, and as illustrated in FIG. 1(C) the openings 91a are provided in both parts of the L-shaped plate at equal intervals in the direction in which the image forming elements are arranged so as to correspond to the incident surface 90a and the emerging surface 90b of each image forming element.
The aperture member 91 having an L-shaped plate configuration in the cross-section as described above shields a relatively large part of the light flux emitted from the light emitting elements array 90, and as a result the transmission efficiency of the image forming elements array 90 is significantly decreased. Particularly, when the thickness of the aperture member 91 is about one n-th of the arrangement pitch of the image forming elements, or the diameter of the openings 91a of the aperture member 91 is smaller relative to the arrangement pitch of the image forming elements, the decrease of the transmission efficiency is remarkable.
The image forming elements array 90 is generally used as an imaging device to form an image of an original document, that is placed on an object plane, on a scanned surface of a photoconductor or surfaces of light receiving elements of a light receiving elements array. In order to form an high quality image of an original document on a scanned surface of a photoconductor, the scanned surface of the photoconductor is required to be appropriately lighted by the light flux reflected from the surface of the original document carrying information of the image. Therefore, when an aperture member is used with an image forming elements array for optimizing the light quantity and for preventing cross talk between the adjacent image forming elements of the image forming elements array, it is important to configure the aperture member so as not to decrease the transmission efficiency of the image forming elements array so that an appropriate quantity of light is maintained.
On the other hand, a light emitting elements array for use in a solid scanning system for a higher resolution, such as 600 dpi or higher, for example, can increase the quantity of an emitting light by increasing the inputting current to the light emitting elements array. However, an excessive inputting current over the limit may damage the light emitting element itself, and further, even when an increased inputting current does not exceed the limit, increase of a current causes increase of emitted heat and power consumption. Therefore, increasing the quantity of an emitting light of a light emitting elements array by increasing the inputting current has some limitation. Accordingly, it is desired that an image forming elements array effectively utilizes a light quantity allowed for a light emitting element of the image forming elements array so as not to decrease the transmission efficiency of the image forming elements array. When the aperture member 91 as described above is used, it is necessary to increase the diameter of the openings 91a of the aperture member 91 and at the same time to reduce the thickness of the aperture member 91 as much as possible. It may be ideal if the aperture member 91 can be eliminated, but, without the aperture member 91, the image forming performance of an image forming elements array may be decreased and as a result, optical beam spots having a good circularity can not be formed reliably.
Further, a solid scanning system using an image forming elements array has a disadvantage in that it is difficult to obtain uniform density in a half-tone image. It has been known that non-uniform density in a half-tone image is caused periodically according to the arrangement pitch of image forming elements of the image forming elements array. A human being is most sensible to non-uniform density occurring in a low frequency area, for example, at about 0.2 to about 1 cycle/mm. The arrangement pitch of image forming elements corresponding to the low frequency cycle of about 0.2 to about 1 cycle/mm is about 1 mm to about 5 mm. Therefore, the arrangement pitch for image forming elements is desired to be made shorter than about 1 mm so that the above low frequency area is avoid.
The image forming elements array used in such a solid scanning system is classified into three types, a lens array having distributed refractive index, a lens array as described for example in Japanese Patent Laid-open Publication No. 6-344598 and an in-prism lens array or a roof-mirror lens array as described for example in Japanese Patent Laid-open Publication No. 5-232400.
A lens array with distributed refractive index is constructed by bundling lenses having distributed refractive index respectively and joining them together using adhesive or other suitable material. Therefore, the optical axis of each lens tends to deviate from that of the other lenses and thereby, the focal point tends to be varied.
A lens array as described for example in Japanese Patent Laid-open Publication No. 6-344598 is not configured as an image erecting system in a direction in which lenses are arranged. Therefore, a shielding mechanism is required for each lens and as a result the optical transmission efficiency is decreased and the light quantity distribution is not uniform.
An in-prism lens array as described for example in Japanese Patent Laid-open Publication No. 5-232400 has a spherical surface and therefore, an optical performance required in an image writing device cannot be obtained. Particularly, the beam spot diameter tends to largely fluctuate.
The applicant of the present application has proposed in Japanese Patent Application No. 10-282295 to provide an image forming elements array that reliably forms an optical beam spot having a relatively small diameter and that has an optical performance required in an image writing device by arranging the image forming elements along a line in an arrangement direction and integrating a plurality of image forming elements having an incident surface, an emerging surface and a pair of reflective surfaces, with the incident surface and the emerging surface having a non-spherical shape.
A solid scanning system using a light emitting elements array and an image forming elements array as described above has been also known to have a disadvantage in that image density is not uniform and white vertical stripes partially appear in a solid black image. It has been confirmed by the applicant that such non-uniformity in the image density in a solid black image is caused by variation of the performance of light emitting elements of the light emitting elements array and/or image forming elements of the image forming elements array.
Further, as described above, the solid scanning system has a disadvantage in that it is difficult to obtain uniform density in a half-tone image, and it has been known that non-uniform density in a half-tone image is caused periodically according to an arrangement pitch of the image forming elements. As also described above, human beings are most sensible to non-uniform density occurring when the arrangement pitch of image forming elements is about 1 mm to about 5 mm (corresponding to the low frequency cycle of about 0.2 to about 1 cycle/mm).
The applicant of the present application has therefore proposed in Japanese Patent Application No. 10-287460 an image forming elements array that reduces non-uniformity in image density occurring partially or periodically by making the arrangement pitch of image forming elements constructing an image forming elements array shorter than about 1 mm.
In order to overcome the problems described above, preferred embodiments of the present invention provide an image forming element array, and an optical printing head and an image forming apparatus including the image forming element array, in which the optical transmission efficiency and the image forming performance are greatly improved and an excellent quality image is produced.
The preferred embodiments further provide an image forming elements array, and an optical printing head and an image forming apparatus using the image forming elements array, in which non-uniformity in the image density is minimized to a level that is not detectable by human beings and thereby, excellent image quality is obtained.
Other preferred embodiments of the present invention provide an image forming elements array, an optical printing head and an image forming apparatus using the array, in which the processing and the mass-production properties are greatly improved.
According to a preferred embodiment of the present invention, an image forming elements array includes a plurality of image forming elements which are arranged in an arrangement direction and integral with each other so as to generate an erected image in the arrangement direction with a common magnification. When the distance along the optical axis from an object plane to an incident surface of the image forming element is represented by L and the distance along the optical axis from the incident surface to the emerging surface is represented by T, a condition 0.20xe2x89xa6T/Lxe2x89xa60.50 is satisfied.
According to another preferred embodiment of the present invention, an image forming elements array includes a plurality of image forming elements which are arranged in an arrangement direction and integral with each other so as to generate an erected image in the arrangement direction with a common magnification. When the distance along the optical axis from the incident surface to the emerging surface is represented by T, and the arrangement pitch of the image forming elements is represented by P, a condition 0.16xe2x89xa6P/Txe2x89xa60.50 is satisfied.
According to another preferred embodiment of the present invention, the arrangement pitch P of the image forming elements is preferably less than about 1 mm. Further, the incident surface and the emerging surface may be non-spherical.
According to another preferred embodiment of the present invention, an optical printing head for forming an optical spot on a scanned surface includes an image forming array having a plurality of image forming elements which are arranged in an arrangement direction and are integral with each other so as to generate an erected image in the arrangement direction with a common magnification. When the distance along an optical axis from an object plane to an incident surface of the image forming element is represented by L and the distance along the optical axis from the incident surface to an emerging surface of the image forming element is represented by T, a condition 0.20xe2x89xa6T/Lxe2x89xa60.50 is satisfied. The optical printing head further includes a light emitting elements array including a plurality of light emitting elements. The image forming elements array is constructed to generate a light flux from each of the plurality of light emitting elements into the optical beam spot on the scanned surface.
According to another preferred embodiment of the present invention, an image forming apparatus includes a scanned surface and an optical printing head for forming an optical beam spot on the scanned surface. The optical printing head includes an image forming array having a plurality of image forming elements which are arranged in an arrangement direction and integral with each other so as to generate an erected image in the arrangement direction with a common magnification. When the distance along an optical axis from an object plane to an incident surface of the image forming element is represented by L and the distance along the optical axis from the incident surface to an emerging surface of the image forming element is represented by T, a condition 0.20xe2x89xa6T/Lxe2x89xa60.50 is satisfied. The optical printing head further includes a light emitting elements array including a plurality of light emitting elements. The image forming elements array forms a light flux from each of the plurality of light emitting elements into the optical beam spot on the scanned surface.
According to another preferred embodiment of the present invention, an image forming elements array includes a plurality of equivalent image forming elements which are arranged in a direction and formed integrated with each other. Each of the plurality of image forming elements includes an incident surface positioned at the incident side, an emerging surface positioned at the emerging side and a pair of reflective surfaces to guide a light flux from the incident surface to the emerging side. The incident and emerging surfaces are respectively non-spherical and the arrangement pitch of the image forming elements is preferably less than about 1 mm.
According to another preferred embodiment of the present invention, in the immediately above image forming elements array, when H represents a lens height, R represents a paraxial radius of curvature, K represents a cone constant, and A, B, C, D represent fixed numbers, a non-spherical shape surface XASP(H) satisfies a condition; XASP(H)=H2/[R+R{1-(1+K)(H/R)2}]+AH4+BH6+CH8+DH10 . . . and a spherical shaps XASP(H) satisfies a condition; XASP(H)=H2/[R+R{1-(H/R)2}], and when a deviation amount (H) of the non-spherical shape XASP(H) from the spherical shape XSPH(H) is represented by; (H)=XASP(H)xe2x88x92XSPH(H), the non-spherical shape XASP(H) may be set such that the deviation amount (H) continuously decreases as the lens height H increases.
Further, the image forming elements array may be configured such that the incident surface and the emerging surface are formed in a substantially same non-spherical shape and each of the plurality of image forming elements forms an erected image in a same magnification in a direction in which the plurality of image forming elements are arranged, and furthermore such that an angle formed by the incident light axis and the emerging light axis is greater than 90 degree.
According to another preferred embodiment of the present invention, an optical printing head for forming an optical spot on a scanned surface includes an image forming elements array having a plurality of equivalent image forming elements which are arranged in a direction and formed integrated with each other. Each of the plurality of image forming elements includes an incident surface positioned at the incident side, an emerging surface positioned at the emerging side and a pair of reflective surfaces to guide a light flux from the incident surface to the emerging side. The incident and emerging surfaces are respectively non-spherical, and the arrangement pitch of the image forming elements is preferably less than about 1 mm. The optical printing head further includes a light emitting elements array including a plurality of light emitting elements. A light flux from each of the plurality of light emitting elements of the light emitting elements array forms the optical beam spot on the scanned surface via at least two image forming elements of the plurality of image forming elements.
According to another preferred embodiment of the present invention, the optical printing head described in the preceding paragraph may include an opening member having a plurality of openings. The opening member may be arranged between the light emitting elements array and the image forming elements array or between the image forming elements array and the scanned surface such that the plurality of openings of the opening member respectively correspond to the image forming elements of the image forming array. Further, the beam spot diameter may be smaller than the arrangement pitch of the light emitting elements.
According to another preferred embodiment of the present invention, an image forming apparatus includes a scanned surface and an optical printing head for forming an optical spot on the scanned surface. The optical printing head includes an image forming elements array having a plurality of equivalent image forming elements which are arranged in an arrangement direction and are integral with each other. Each of the plurality of image forming elements includes an incident surface positioned at the incident side, an emerging surface positioned at the emerging side and a pair of reflective surfaces to guide a light flux from the incident surface to the emerging side. The incident and emerging surfaces are respectively non-spherical, and the arrangement pitch of the image forming elements is preferably less than about 1 mm. The optical printing head further includes a light emitting elements array including a plurality of light emitting elements, and a light flux from each of the plurality of light emitting elements of the light emitting elements array forms the optical beam spot on the scanned surface via at least two image forming elements of the plurality of image forming elements.
Other features, elements and advantages of preferred embodiments of the present invention will be disclosed in the detailed description of preferred embodiments below.